1. Field of the Invention
The present invention relates to an exposure device for exposing a photoconductive member in an image forming apparatus such as a copier or a printer, and an image forming apparatus provided with such an exposure device.
2. Description of the Related Art
A light scanning unit (LSU: laser scanner unit) is known as an exposure device of an electrophotographic image forming apparatus such as a copier and a printer. In this LSU, a light emitted from a laser based on image information is caused to be incident on a polygon mirror (rotary multifaceted mirror) rotated at high speed by a polygon motor via a lens, and the resulting reflected light is caused to irradiate the surface of a uniformly charged photoconductive drum via another lens, thereby scanning and exposing the surface of the photoconductive drum. In this way, an electrostatic latent image corresponding to the image information is formed on the surface of the photoconductive drum.
Since the polygon motor and the polygon mirror rotate at high speeds in the LSU as described above, noise and heat are generated. The noise leads to the deterioration of a workplace environment, whereas the heat generation causes the nonuniform rotation of the polygon mirror and distortions in optical devices such as lenses, thereby causing an exposure failure.
Accordingly, an exposure device covering a polygon motor and a polygon mirror using a double cover; the one cooling a polygon motor and a polygon mirror by an air flow created by a fan (e.g. Japanese Unexamined Patent Publication No. 2000-194250), and the one cooling a polygon motor and a polygon mirror by a liquid (e.g. Japanese Unexamined Patent Publication No. 2005-31160) have been proposed.
However, the above exposure device covering the polygon motor and the polygon mirror using the double cover can reduce the noise, but is rather disadvantageous in light of temperature rise since the heat remains in an inner casing. The exposure devices cooling the polygon motor and the polygon mirror by an air flow or liquid have a problem of complicated constructions.